12 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



83. If a ligature be made round the bark, below a branch, 

 the part above the ligature swells, that below it does not 

 swell. 



84. Therefore the matter which causes the increase of 

 Exogenous plants in diameter descends. 



85. If a growing branch is cut through below a leaf-bud, 

 that branch never increases in diameter between the section 

 and the first bud below it. 



86. The diameter of all Exogenous stems increases in each 

 species in proportion to the number of leaf-buds developed. 



87. The greater number of leaf-buds above a given part, 

 the greater the diameter of that part ; and vice versa. 



88. In the spring the newly forming wood is to be traced 

 in the form of organic fibres descending from the leaf-buds ; 

 that which is most newly formed lying on the outside, and 

 proceeding from the most newly developed buds. 



89. Therefore the descending matter, by successive addi- 

 tions of which Exogenous plants increase in diameter, pro- 

 ceeds from the leaf-buds. 



90. Their elongation upwards gives rise to new axes, with 

 their appendages ; their elongation downwards increases the 

 diameter of that part of the axis which pre-existed, and pro- 

 duces roots. 



91. Roots, therefore, in all cases, should consist of exten- 

 sions of woody tissue ; and this is conformable to observation. 



92. Hence, while the stem is formed by the successive 

 evolution of leaf-buds, the root, which is the effect of that 

 evolution, has no leaf-buds. 



93. The leaf-buds thus successively developed are firmly 

 held together by the medullary system of the stem, which 

 proceeds from the bark inwards, connecting the circumference 

 with the centre. 



94. The stem varies in structure in four principal ways. 



95. It is either formed by successive additions to the out- 

 side of the wood, when it is called Exogenous^ ; or by suc- 

 cessive additions to its centre, when it is called Endogenous^ ; 

 or by the union of the bases of leaves, and by addition to the 

 point of the axis, or by simple elongation or dilatation where 

 no leaves or buds exist ; this is called Acrogenous. 



96. In what are" called Dictyogens^, the stem has the 



